As is known, harmonic mixing has been used in the past primarily at the higher millimeter wave frequencies where reliable, stable local oscillator sources are either unavailable or prohibitively expensive. However, the conversion loss obtained by harmonic mixing has been typically 3 to 5 db greater than that which can be obtained by fundamental mixing at the same signal frequency. Analysis shows that such a large degradation should not exist, but it assumes that fundamental mixing between the signal and local oscillator is suppressed. Fundamental mixing will, however, take place unless the harmonic mixer provides a reactive termination for these mixer products. In general, that is difficult to accomplish (e.g., In the case of second harmonic mixing, the fundamental mixing difference frequency (f.sub.s - f.sub.LO) is close to the local oscillator frequency).